The present disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides for use of swellable material in an annular seal element to prevent leakage in a well.
Leak paths can sometimes arise in cemented intervals due to poor cement bonding to a surrounding earth formation surface, incomplete mud filter cake removal prior to placing cement in the interval, subsidence and compaction. In some circumstances, the cement will not bond properly to the interior surface of an outer casing or formation surface because of incomplete drilling fluid removal from the surface, presence of a filter cake on the surface or a film of drilling mud on the surface. In horizontal wells, a fluid channel may develop on a high side of the wellbore, due to (but not limited to) fluid migrating out of the cement slurry or density differences of the different liquid materials in the wellbore.
In addition, situations can arise in which the cement takes an initial bond to the surface of the casing or wellbore, but then de-bonds (separates) from the surface at some point in the future. These situations can be due to, for example, reservoir subsidence, tectonic plate movement, fluctuating temperatures, fluctuating pressures and changes in wellbore stresses.
When these situations arise, and there is no effective seal along the interval (e.g., in an annulus between two casing strings, or between a casing string and the inner surface of the wellbore), fluids can migrate from one reservoir or zone to another, or to the surface. Uncontrolled flow between reservoirs is often called an “underground blowout” and is highly undesirable. Reservoir fluids (liquids and/or gases) unintentionally flowing to the surface (e.g., between casing strings) is often called “casing pressure.” If the pressures exerted by the fluids persist for extended periods, then it is often called “sustained casing pressure.”
Currently, there is no completely satisfactory solution to these problems. It is known to use a swellable packer along a cemented interval so that, if the cement leaks, the packer can swell and close off the annulus, but the packer is enclosed in the cement and cannot reliably close off a fluid channel in the cement itself. The swellable element will not seal the channel unless there is direct contact with the channel and the fluid therein. It is also known to mix particles of swellable material in the cement slurry, but this method results in a relatively small effective volume change, which may not be sufficient for sealing off larger leak paths.
Therefore, it will be appreciated that improvements are needed in the art of preventing leakage in a subterranean well.